The present invention relates to wavelength monitoring of multi-wavelength optical signals in optical communications systems.
High data rates are achieved in optical communications systems using wavelength division multiplexed (WDM) signals. WDM signals are multi-wavelength optical signals that include multiple channel signals, each at a predefined channel wavelength. In a WDM-based optical communications system, the channel signals are generated by a series of modulated transmitters and the channel signals may be separated by 25 GHz to 200 GHz within the 1528-1565 nm wavelength range. The 1528-1565 nm wavelength range is within the flat gain region of a erbium doped fiber amplifier (EDFA), a critical component of modern optical communications systems. Performance of an optical communications system can be verified by monitoring the wavelength, power, and signal-to-noise ratio of each of the WDM channel signals. Wavelength monitoring of the channel signals assures that deviations from the predefined channel wavelengths due to drifts or instabilities in the transmitters are detected. Wavelength monitoring also verifies that channel signals subsequently added to the multi-wavelength optical signals are within specified wavelength deviations of the precisely defined channel wavelengths.
While wavelength monitoring can be performed using commercially available optical spectrum analyzers (OSAs), OSAs that include motors to rotate optical gratings or optical filter elements may be too large to be integrated within an optical communications system. In addition, the mechanisms required to rotate the optical gratings present many operational and reliability problems. OSAs based on tunable Fabry-Perot interferometers are also utilized for wavelength monitoring. Disadvantages of simple tunable Fabry-Perot filters include the limited tuning range and poor resolution that limits channel differentiation.
OSAs based on InGaAs/InP photodetector arrays have a small physical size, but are expensive to manufacture, making it economically unfeasible to incorporate the OSAs into most optical communication systems. For example, an OSA that utilizes a fixed optical grating and a linear photodetector array to detect multiple wavelengths (i.e., channels) must have at least one individually addressable pixel element for each channel that is to be identified. In a dense wavelength division multiplexed (DWDM) system of, for example, 1,000 channels, a complex and costly detector array having at least 1,000 individually addressable pixels is required.
Optical measurement instruments other than OSAs, such as multi-wavelength meters, are also used to monitor the wavelength of channel signals. However, many of these types of instruments are physically large and expensive to manufacture.
As a result of the disadvantages of prior art wavelength monitors, what is needed is a wavelength monitor that can be tuned over the broad wavelength range commonly used for WDM optical communications systems and with the high resolution required for narrow channel spacing in an environment that requires no moving parts.
A method and a system for monitoring specific channels in a WDM system involve splitting a WDM signal into multiple parallel signals, filtering the parallel signals with individually tunable filters in order to pass specific channels through each filter, and then detecting the presence of passed channels with dedicated detectors that correspond to the individually tunable filters.
In a preferred embodiment, the initial WDM signal is demultiplexed by wavelength into transmission groups, with each transmission group including multiple channels in a continuous wavelength range. The transmission groups are then filtered by group-specific filters which are preferably formed utilizing semiconductor wafer processing techniques. Each of the group-specific filters can be individually tuned to pass a specific channel from the transmission group with which the filter is associated. The group-specific filters are preferably tunable over channel ranges that correspond to the channel ranges of the respective transmission groups. That is, each filter is tunable over a different channel (i.e., wavelength) range such that the entire wavelength range of the optical system is covered by the combination of the filters. Specific channels that pass through the filters are detected by simple low cost photodetectors. By demultiplexing the WDM signal into parallel transmission groups, WDM signals having a broad wavelength range can be monitored with a combination of relatively simple filters and photodetectors. This reduces significantly the requirements on the filter and detector array.
The preferred wavelength monitoring system includes an input section, a splitter, tunable filters, and detectors. The input section enables the multiplexed optical signals to enter the splitter. The input section is preferably an optical fiber, but may be free space or a device such as an optical V-groove array. The splitter may include a conventional power splitter that divides the energy of optical signals into transmission groups, without regard to wavelength, for transmission over multiple optical paths. In a preferred embodiment, the splitter is a demultiplexer that divides the optical signals by wavelength into transmission groups that include subsets of the original wavelength range. The tunable filters are preferably vertical cavity filters that are fabricated onto monolithic substrates utilizing photolithographic processes. The tunable filters are individually tunable over wavelength ranges that may include the entirety of the original wavelength range of the particular optical communications system, or that preferably include subsets of the original wavelength range. Each filter has a passband that is equal to or less than the bandwidth of a single channel, depending on the application. The individually tunable filters are calibrated so that the particular passband of each filter can be determined at any particular time. The detectors include conventional photodetectors that generate electrical current in response to the presence of optical energy. Because the tunable filters pass optical energy of known wavelengths, the photodetectors are not required to be channel sensitive.
In a preferred operation, WDM optical signals are input into the demultiplexer through an input fiber. The WDM signals are demultiplexed into optical signals of different wavelength ranges. The demultiplexed optical signals are then transmitted to respective vertical cavity filters via optical paths. Each of the vertical cavity filters is tunable over a unique wavelength range, with the wavelength ranges generally corresponding to the demultiplexed wavelength ranges of the WDM signals. The vertical cavity filters are individually tuned to selectively pass one optical channel, while blocking transmission of all other optical channels.
Optical energy that passes through the vertical cavity filters is incident on the corresponding photodetectors. The detection of optical energy by the photodetectors is correlated to the wavelength of optical energy (i.e., channel) that was allowed to pass through the respective filter at the corresponding time. The vertical cavity filters can be successively tuned to sweep across all of the wavelengths in the respective wavelength ranges in order to determine the presence or absence of channels within each range.
By dividing the multiplexed incoming optical signals into transmission groups, the individually tunable filters in the wavelength monitor need only be tunable over a portion of the total wavelength range of the incoming WDM signals. In addition, because each of the optical filters passes only one channel at a time, each photodetector in the wavelength monitor can be an unsophisticated and inexpensive single-cell photodetector. In contrast, a conventional monitoring system having a fixed grating typically requires a large detector array that includes an individually addressable pixel for each monitored channel.